PORTSMOUTH — The scientists charting the world's seafloors have come a long way from the days when sailors would head out on ships and tie a lead sinker to a line to measure ocean depths.

The use of rapid-fire sonar technologies and computer programs developed at the University of New Hampshire has revolutionized seafloor mapping and drastically improved the accuracy of nautical charts.

And while the science behind it is complicated, former UNH graduate student and research ship commander Shepard Smith said his days out on a hydrographic survey vessel provide the data necessary to create "road maps for the ocean."

Smith and his crew of researchers from the National Oceanic and Atmospheric Administration (NOAA) steamed into Portsmouth aboard the 205-foot Thomas Jefferson on Friday after conducting a survey of the seafloor off the coast of York, Maine.

The brief NOAA study of a small area off the Maine coastline used multiple beam sonar soundings to send thousands of "pings" to the ocean floor that has painted a new picture of the depth and topography of a small portion of the coast. The last survey is believed to have taken place when rope and not sonar was used to gauge depth.

Smith said the quick survey showed an area whose minimum depth on maps was long marked at 32 feet is actually only 20 feet, thus proving the importance of updating nautical charts for the safety of ships traveling America's coastlines.

The Norfolk, Virginia-based Thomas Jefferson is one of three NOAA hydrographic survey ships whose task is to keep updating nautical maps to account for errors in the old methodology and to keep up with a seafloor that Smith said is constantly changing.

NOAA operates under the federal Department of Commerce and is responsible for studying everything from weather to ocean depths.

Smith's ship is the only one of its kind that studies the ocean floor on the East Coast with the other NOAA survey vessels focusing their efforts off the Alaska coastline. He said the boats rarely travel far out to sea and focus their efforts on mapping the shallower coastline areas to assure the seafloor depths and obstructions at port entrances are properly understood to provide for safe passage.

Seafloor mapping in America was first initiated under President Thomas Jefferson, who pushed for a survey of the country's East Coast in the early 1800s, which took more than 40 years to complete.

Smith said the old method of mapping the ocean's bottom involved a ship and crew dropping "lead-lines" and measuring the length of a rope from the bottom of a weight when it hit the seafloor.

It might be surprising to some that many of the current nautical charts include depths taken in surveys before the existence of sonar and NOAA has continuously been working to use advanced sonar applications to improve the accuracy of charts.

Smith, a Belfast, Maine native, said NOAA's hydrographic exploration has a close tie to New Hampshire's Seacoast through the UNH Center for Coastal and Ocean Mapping, considered a forerunner in seafloor mapping technology.

Smith attended the UNH program as a graduate student and said experts from the UNH hydrographic center have helped develop numerous technologies that help them process immense amounts of data collected through sonar devices placed on NOAA ships.

The ship and its two deplotyable 31-foot survey boats are all equipped with sonar that send thousands of signals out to determine water depths with the technology also being capable of determining whether the bottom consists of muddy sediment or rocky mounds.

Smith said the updated information on seafloor depths and composition has numerous applications.

The vessels head to particular areas of the ocean and make numerous passes to chart the ocean's floor.

"It gives us a picture we can use not only for charting, but for fisheries habitat and ocean geology," Smith said.

The Thomas Jefferson carries a crew of NOAA officers and researchers who study their findings and send the data out so charts can be updated.

Smith said applications developed at UNH, including the "Combined Uncertainty and Bathymetry Estimate" or CUBE algorithm, are used with computer programs to provide three-dimensional maps of the ocean's floor, which is always changing and littered with shipwrecks.

The commander recalled one instance in which he was working on a NOAA vessel that discovered a sunken tugboat in Chesapeake Bay that was unknown at the time and sitting only 30 feet under the water's surface near a popular shipping route.

The Thomas Jefferson has located dozens of shipwrecks during its travels and helped provide updated seafloor depths so they might be used to update charts.

NOAA survey data could also be used to help ensure homeland security by providing a clear and updated picture of the seafloor in various ports. Such information would be used to search for underwater explosive devices in the event the country came under attack and an enemy placed underwater mines in shipping lanes.

Smith said NOAA is currently working to build a new state-of-the-art, dual-hulled survey ship that will be kept in New Castle so that its home port might be close to a UNH program that continues to facilitate more advanced hydrographic mapping. He said NOAA anticipates taking possession of the boat and adding it to the fleet this coming spring.

"We want to continue to have a close relationship," Smith said.

And while the Thomas Jefferson was set to leave Portsmouth on Monday afternoon, its work will continue with no end in sight.